The subtle decline in our sense of smell as we age may elude our awareness but can herald the early stages of neurological conditions.
Assessing whether one's sense of smell is functioning properly might seem straightforward, yet many individuals might not accurately gauge their own olfactory abilities. It's not uncommon for people to claim their sense of smell is unimpaired when, in reality, it might be significantly compromised. Some individuals might even be anosmic—entirely lacking a sense of smell—without their awareness.
Anosmia refers to the total absence of smell perception and it is a sensory impairment, akin to sensory losses such as blindness and deafness. It belongs to a broader group of olfactory disorders, which include varying levels of reduced smell sensitivity (hyposmia) and the experience of phantom odors - “ghost smells” (phantosmia) - detecting an often unpleasant odor when no corresponding source is present. Anosmia's causes vary. A small number of anosmics are congenitally anosmic due to genetic or developmental factors; with their prevalence in the population being estimated at 1 in 10,000.
In contrast, the majority of olfactory disorders are acquired, either temporarily or permanently, often due to nasal passage blockages or inflammation, or brain injury. According to the National Health and Nutrition Examination Survey (NHANES) approximately 12.4% of American adults experience some form of olfactory impairment, with around 3.2% suffering from anosmia or severe smell impairment. Similar to other senses such as sight or hearing, the sense of smell tends to deteriorate with age. In fact, our olfactory capabilities reach their peak around the early thirties, maintaining stability for about three decades, and then decrease rapidly after the age of 60. By the age of 80, nearly 40% of individuals may experience a significant or complete loss of their sense of smell.
Today there exist several types of smell test kits that offer a means to quantitatively assess olfactory capabilities. These kits typically require participants to identify familiar scents, such as cinnamon or banana, or to differentiate between scented samples and unscented controls. Large-scale studies utilizing these tests have consistently found a marked lack of correlation between individuals' self-assessed olfactory function prior to the test and their actual performance on these tests. For example, in one study involving about 9,000 healthy participants of all ages who reported that their sense of smell was normal, 3.4% were actually found to be anosmic, according to the smell test. Another study, focusing on adults aged 57–85, found that only about one quarter of those classified as having an impaired sense of smell were aware of their impairment. In other words, in many cases we are simply unaware of our own potential smell disorders.
Could a Diminished Sense of Smell Signal Cognitive Decline? | Illustration: Shutterstock
Right Under Your Nose
Contemplating the reasons behind the unreliability of self-reported assessments concerning one's sense of smell is intriguing. One contributing factor could be the overly broad nature of conventional survey inquiries, such as "Is your sense of smell normal?" A more nuanced approach, involving questions regarding changes in individuals’ ability to discern specific odors over time, could improve the reliability of responses. In the NHANES survey, the subjects were asked detailed questions about their sense of smell in order to bridge this gap; nevertheless, even this survey found a significant discrepancy between subjective self-reports and objective smell test results.
Another factor that may explain the discrepancy could be the gradual decline of our olfactory faculties as we age. The imperceptible nature of this process might obscure one's perceptual changes, rendering them less noticeable. Indeed, those experiencing sudden smell loss due to conditions such as sinusitis were found to report their symptoms much more accurately, suggesting that rapid changes are more readily acknowledged. Similarly, individuals with severe olfactory deficits are generally more aware of their condition compared to those with milder impairments. In addition, age-related cognitive decline and memory loss may further contribute to the lack of awareness of one’s scent perception. However, despite the correlation between olfactory capabilities and memory and awareness of the sense of smell, the discrepancy between self-perception and actual olfactory ability remains notable, even among cognitively healthy individuals.
A Telltale Sign
We can also ask, why does this matter at all? If unawareness of olfactory impairment doesn't impact daily life, is it truly problematic? Well, the answer is complex. First, a lack of awareness of smell disorders could amplify associated risks, like ignoring warning scents in the environment, such as smoke or gas, or failing to detect spoiled food. Second, accurately assessing a patient's olfactory abilities is crucial for diagnosing conditions where anosmia or reduced smell sensitivity are one among many symptoms. Furthermore, in some neurological diseases, including Alzheimer’s and Parkinson’s, olfactory senses tend to diminish relatively early in the disease's progression, thus providing an opportunity for early diagnosis. Thus, healthcare providers must rely on objective smell testing rather than patient self-reporting to consider olfactory impairment in their diagnostic and treatment strategies.